In recent years, environmental problems such as destruction of the ozone layer, global warming, environmental pollution, and air pollution are being increasingly addressed. At the same time, for environmental protection, the governments of many countries have imposed various controls, for instance, in the US the National VOC [Volatile Organic Compound] Emission Standard (Section 183 e), in the EU countries Council Directive (1999/13/EC) and in some accession countries, e.g. Hungary (10/2001 Governmental Decree), have been promulgated to regulate activities in areas of concern. In such circumstances, releasing organic solvents in the air has become a significant problem and, consequently an attempt to use less or no solvents has become increasingly common in many industries. Also in the coating industry, powder coatings are being tested as substitutes for conventional solvent-type coatings. Improvements in the coat film performance of high-solid, or advanced high-solid coatings are being developed to meet requirements of environmental protection and good technical applicability. The development of new polymers with designed structure and architecture has, consequently, become an intensively studied field of material science.
It is known in the art to use core/shell, star and microgel polymers to combine mechanical and thermal properties of polymeric particles. The core is formed by cross-linking mono- and/or di- or higher multifunctional vinyl monomers. Then in a second stage the outer shell is formed from monovinyl and di- and/or multi-vinyl monomers that optionally may be crosslinked. Macromolecules typically exist in a solution phase as a sol macromolecular colloid system. These systems are usually prepared in an aqueous medium as an emulsion polymerization. The polymerization of the monomers with multiple vinyl functionality results in gelation of the whole mixture.
The following patents further describe the technological background of the present invention:
Fujii et al. in U.S. Pat. No. 5,298,559 describe a multi-layered polymer system polymer having core layer of an aromatic vinyl polymer, an intermediate layer of a butadienic rubbery polymer and an outer layer of an aromatic vinyl glassy polymer, that provides a thermoplastic resin composition excellent not only in impact strength, especially in impact strength at low temperatures.
Oshima et al. in U.S. Pat. No. 5,324,780 prepare core-shell polymers comprising a core phase which is a rubbery polymer and a shell phase which is a glassy polymer with an unsaturated dicarboxylic acid or its mono-alkyl ester as a constituent thereof, wherein the toluene-soluble fraction of the core-shell polymer accounts for not more than 10% by weight. The resin compositions and molded articles which comprise the core-shell polymer as an impact modifier have good features, especially a high impact strength at temperatures ranging from room temperature to −30.degree. C.
Eisenhart et al. in U.S. Pat. No. 5,451,641 describe a polymeric thickener which consists of multi-stage polymer particles comprising at least one hydrophobically-modified, ionically-soluble polymer stage polymerized from hydrophobic monomer and ethylenically-unsaturated monomers. The grafting monomer may be, among others, an unsaturated carboxylic acid allyl ester, such as allyl acrylate, allyl methacrylate, diallyl maleate, diallyl fumarate, diallyl itaconate, etc. In particular, allyl methacrylate is preferred. Such crosslinking monomer and grafting monomer are used each in an amount within the range of 0.01 to 5% by weight, preferably 0.1 to 2% by weight, based on the total monomer amount for the core.
Takeuchi et al. in U.S. Pat. No. 5,453,458 describe a two and multistage polymerization forming a core-shell polymer which core is composed essentially of a cross-linked polymer of styrene. The core-shell polymer of the invention can be produced by a multi-stage seed emulsion polymerization method or a multi-stage suspension polymerization method. In the first-stage polymerization, they use a cross-linking monomer as a monomer in an amount of not more than 5% by weight, preferably in an amount of not more than 2% by weight, based on the total of the core forming monomers used in the first-stage polymerization. The polymerization described here gives only a lightly crosslinked core with a limited impact properties.
Chandran et al. in U.S. Pat. No. 6,165,563 describe star-branched polymers containing pendent olefinic groups which have been crosslinked using actinic radiation, and the use of these polymers in adhesives and coating applications.
Letchford et al. in U.S. Pat. No. 6,221,991 disclose an anionic polymerization to provide novel polar polymers, including functionalized, telechelic, heterotelechelic, and multi-branched or star methacrylate and acrylate polymers, and processes for preparing the same. The novel polymers have applications in a variety of areas, including use in low VOC coatings, adhesives, and as viscosity index (V.I.) improvers for lubricants. The invention also provides processes for anionic polymerization of polar monomers to produce the polymers of the invention. These polymers are prepared from protected functionalized initiators which are reacted with an appropriate diaryl alkenyl group, such as 1,1-diphenylethylene, to provide a stabilized carbanion. A polar monomer, preferably methyl methacrylate, is polymerized in the presence of the initiator to provide a living anion.
Blankenship et al. in U.S. Pat. No. 6,252,004 disclose a process for preparing emulsion polymer particles providing an aqueous emulsion of a multistage core-shell polymer with a hydrophilic core. The process produces multistage polymers having low dry-bulk density useful in coating compositions such as paints and paper coatings.
Solomon et al. in U.S. Pat. No. 6,300,443 describe a process for preparing polymeric microgels comprising reacting an alkoxyamine with an unsaturated monomer composition comprising a cross-linking agent comprising at least two double bonds and optionally one or more further monomers selected from monounsaturated monomers and conjugated diene monomers.
Lubnin et al. in U.S. Pat. No. 6,316,107 disclose an emulsion or suspension polymer comprising a vinyl chloride polymeric core and an acrylic ester-acrylonitrile polymeric shell. The emulsion polymer was preferably prepared using a two-stage process. In the first stage, a vinyl chloride monomer was polymerized or copolymerized to form a first phase of a polymeric hard core having a relatively high chlorine content. In a second stage, the soft acrylic ester-acrylonitrile copolymer was made in situ in a reaction mixture comprising the first phase. The product provides both flame retardancy and low minimum film-forming temperature (MFFT), and is useful in a variety of coating and binding applications.